Tracking systems commonly determine the position of an object by analyzing its image with a segmented detector having a multiplicity of array subelements that are approximately equal in size and shape. Optical systems typically form an image on a spectrally selective semiconductor array of subelements, in either the visible, infrared, or ultraviolet spectral ranges. The high resolution of optical systems enables the precise setting of detector subelement size, shape, and placement. Most commonly, square detector subelements are used in such systems, although other symmetric shapes, such as mosaics of regular hexagons, have been used in detector arrays. Non-optical systems can use the same imaging principles. Radar systems form an image by antenna beam patterns, and the detector is a frequency-selective receiver. Acoustic systems form an image on a sound-selective sensor.
In each case, the image is divided among multiple detector array subelements, either by simultaneously sensing on a fixed detector array, or by sequentially sensing as the scene is moved over the detector. This image motion can be imparted by physical motion of the scene, the entire sensor, the detector, the imaging means, or other sensor-internal mechanism. The image division among multiple subelements also can be imparted with no physical motion, by electronic processing that subdivides a scene image, or by electronic scanning that moves sub-images across a large scene image.
In one common approach, where the image is not much larger than a detector subelement, the location of an object is determined relative to the detector centerline by analyzing the difference in signal between adjacent subelements. This can be done in one dimension, with two array subelements adjacent in a side-by-side relation. The “left-right” position is the simple difference of the signal from the “left” subelement minus the signal from the “right” subelement. Usually this difference signal is normalized by dividing by the sum of the “left” and “right” signals, in order to give a position indication that does not vary with signal strength. This same principle may be applied in two dimensions, using four detector subelements in a symmetrical quadrant array, with the subelements pair-wise differenced. For example, “up-down” signals are generated by the difference between the sums of the signals from the two “upper” subelements, minus the sum of the signals from the two “lower” subelements. Such techniques are common in monopulse radar sensors that are used for missile guidance and other applications. These techniques are also used in optical trackers and acoustic sensors. Such location signals are often used by a tracking mechanism, to set the aim point at this centerline location.
In another common approach, where the image covers several array subelements, the center or centroid of the image is determined. This approach is most common in optical systems, which typically use large arrays of detector subelements to create a scene image. The location determination then is made by locating the center or centroid within this array. This approach determines the position over a wider range of displacements than is possible with only two or four detector subelements, yet maintains the same positional accuracy.
The accuracy of the determination of the location of the feature is a crucial concern in such sensors. The spatial location of the image on the detector array may be translated geometrically into an angle of the feature relative to a reference axis of the sensor, such as the boresight of the platform that carries the sensor.
An important unsolved problem is that there may be substantial positional ambiguity in determining the image location on the array of subelements, which translates into a substantial angular ambiguity in determining the location of the feature that produces the image. There is an ongoing need for an improved approach for segmented-array detectors to achieve increased accuracy in the determination of the location of the image of the feature without sacrificing sensitivity. The present invention fulfills this need, and further provides related advantages.